Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||20h 06m 15.457s|
|Declination||+44° 27′ 24.61″|
|Apparent magnitude (V)||±0.017+11.705|
|Evolutionary stage||Main sequence|
|Spectral type||F3 V/IV|
|B−V color index||0.557|
|V−R color index||0.349|
|R−I color index||0.305|
|J−H color index||0.212|
|J−K color index||0.264|
|Absolute magnitude (MV)||3.08|
|Luminosity (bolometric)||4.7 L☉|
|Luminosity (visual, LV)||5 L☉|
|Surface gravity (log g)||±0.2 4.0cgs|
|Rotation||±0.0001 days 0.8797|
|Rotational velocity (v sin i)||±4 km/s 84|
KIC 8462852 (eponymously Tabby's Star after lead author Tabetha S. Boyajian, or WTF Star, formally for "Where's The Flux?", but also a reference to an expression of disbelief) is an F-type main-sequence star located in the constellation Cygnus approximately 454 parsecs (1,480 ly) from Earth. Unusual light fluctuations of the star were discovered by citizen scientists as part of the Planet Hunters project, and in September 2015 astronomers and citizen scientists associated with the project posted a preprint of a paper on arXiv describing the data and possible interpretations. The discovery was made from data collected by the Kepler space telescope, which observes changes in the brightness of distant stars in order to detect exoplanets.
KIC 8462852 is so far the only known star with such behavior among the 150,000 stars monitored by the Kepler mission.
Several hypotheses have been proposed to explain the star's large irregular changes in brightness as measured by its unusual light curve. The leading hypothesis, based on a lack of observed infrared light, is that of a swarm of cold, dusty comet fragments in a highly eccentric orbit. Many small masses in "tight formation" orbiting the star have also been proposed. The changes in brightness could be signs of activity associated with intelligent extraterrestrial life building a Dyson swarm. The SETI Institute's initial radio reconnaissance of KIC 8462852, however, found no evidence of technology-related radio signals from the star.
KIC 8462852 in Cygnus is located roughly halfway between the major visually apparent bright stars Deneb (α Cyg, α Cygni, Alpha Cygni) and Delta Cygni (δ Cyg, δ Cygni) to the eye as part of the Northern Cross. KIC 8462852 is situated south of Omicron¹ Cygni (ο¹ Cygni, 31 Cygni), and northeast of the star cluster NGC 6866. While only a few arcminutes away from the cluster, it is unrelated and closer to the Sun than it is to the star cluster.
History of observations
The main source of the information about the luminosity fluctuations of KIC 8462852 is the Kepler space observatory, which was launched in 2009.
Observations of the luminosity of the star by the Kepler space telescope show small, frequent, non-periodic dips in brightness, along with two large recorded dips in brightness appearing to occur roughly 750 days apart. The amplitude of the changes in the star's brightness, and the aperiodicity of the changes, mean that this star is of particular interest for astronomers. The star's changes in brightness are consistent with many small masses orbiting the star in "tight formation".
The first major dip, on 5 March 2011, obscured the star's brightness by up to 15%, and the other (on 28 February 2013) by up to 22%. In comparison, a planet the size of Jupiter would only obscure a star of this size by 1%, indicating that whatever is blocking light during the star's major dips is not a planet, but rather something covering up to half the width of the star. Due to the failure of two of Kepler's reaction wheels, the star's predicted 750-day dip around April 2015 was not recorded; further observations are planned for May 2017.
- Light curves
In addition to the day-long dimmings, a study of a century's worth of photographic plates suggest the star has gradually faded from 1890 to 1989 by about 20%, which would be unprecedented for any F-type main sequence star. However, teasing accurate magnitudes from long term photographic archives is a complex procedure, requiring adjustment for equipment changes, and is strongly dependent on the choice of comparison stars. A contrasting study, examining the same photographic plates, concluded that the possible century-long dimming was likely a data artifact, and not a real astrophysical event. It is hoped that reviews of additional photographic archives can settle this issue.
Based on the star's spectral and star type, the star's changes in brightness could not be attributed to intrinsic variability, so a few hypotheses have been proposed involving material orbiting the star and blocking its light, but none of these fully explain the observed data.
Some of the proposed explanations involve instrument or data artifacts, variable B(e) star, interstellar dust, a series of giant planets with very large ring structures, a recently captured asteroid field. and undergoing Late Heavy Bombardment.
Younger star with coalescing material around it
Astronomer Jason Wright (who was consulted by Boyajian) and others who have studied KIC 8462852 have suggested in a follow-up paper that if the star is younger than its position and speed would suggest, then it may still have coalescing material around it.
A 0.8–4.2 micron spectroscopic study of the system using the NASA Infrared Telescope Facility found no evidence for coalescing material within a few astronomical units of the mature central star.
Planetary debris field
High-resolution spectroscopy and imaging observations have also been made, as well as spectral energy distribution analyses using the Nordic Optical Telescope in Spain. A massive collision scenario would create warm dust that glows in infrared wavelengths, but there is no observed excess infrared energy, ruling out massive planetary collision debris. Other researchers think the planetary debris field explanation is unlikely, given the very low probability that Kepler would ever witness such an event due to the rarity of collisions of such size.
A 0.8–4.2 micron spectroscopic study of the system using the NASA Infrared Telescope Facility found no evidence for hot close-in dust, circumstellar matter from an evaporating or exploding planet within a few astronomical units of the central star
A study of past infrared data from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer found no evidence for an excess of infrared emission from the star, which would have been an indicator of warm dust grains that could have come from catastrophic collisions of meteors or planets in the system. This absence of emission supports the hypothesis that a swarm of cold comets on an unusually eccentric orbit could be responsible for the star's unique light curve, but more studies are needed.
A cloud of disintegrating comets
One proposed explanation for the star's odd reduction in light is that it is due to a cloud of disintegrating comets orbiting the star elliptically. Under this scenario, gravity from a nearby star may have caused comets from the star's Oort cloud to fall in toward the star. Evidence supporting this hypothesis includes a red dwarf within 132 billion kilometers (885 AU) of KIC 8462852. However, the notion that disturbed Oort cloud comets orbiting elliptically close to the star could exist in high enough numbers to obscure 22% of the star's observed luminosity has been doubted.
Submillimetre wavelength observations searching for farther-out cold dust in the system's Kuiper Belt suggest that a distant "catastrophic" planetary disruption explanation is unlikely; the possibility of a disrupted Kuiper Belt scattering comets into the inner system is still to be determined.
Astronomer Jason Wright and others who have studied KIC 8462852, hypothesized that the objects eclipsing the star could be parts of a megastructure made by an alien civilization, such as a Dyson swarm, a hypothetical structure that an advanced civilization might build around a star to intercept some of its light for their energy needs. Due to extensive media coverage on this matter, KIC 8462852 has been compared by Kepler's Steve Howell with KIC 4110611, another star with an odd light curve (which proved, after years of research, to be a part of a five-star system). Regarding the current light curve data of KIC 8462852, Wright has emphasized the importance of upcoming spectral studies. According to Wright, the likelihood of extraterrestrial intelligence being the cause of the dimming is very low; however, the star is an outstanding SETI target because natural explanations have yet to fully explain the dimming phenomenon.
On 19 October 2015, the SETI Institute announced that it had begun using the Allen Telescope Array to look for radio emissions from possible intelligent extraterrestrial life in the vicinity of the star. After an initial two-week survey, the SETI Institute reported in November 2015 that it found no evidence of technology-related radio signals from the star system KIC 8462852. In February 2016 another SETI related study, one using archival VERITAS gamma ray observatory observations from 2009 to 2015, found no evidence of pulsed optical beacons associated with KIC 8462852.
Many optical telescopes are monitoring KIC 8462852 in anticipation of another multi-day dimming event, with planned follow-up observations of a dimming event using large telescopes equipped with spectrographs to determine if the eclipsing mass is a solid object, or is composed of dust or gas. Additional follow-up observations may involve the ground-based Green Bank Telescope, the Very Large Array Radio Telescope, and future orbital telescopes dedicated to exoplanetology such as WFIRST, TESS, and PLATO.
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